This invention relates to an aluminum alloy for use in a brazed assembly as a core material in brazing sheet, to the use of the aluminum alloy as core material of a brazing sheet in a brazed assembly, to the use of the aluminum alloy as fin stock material, to a method for manufacturing a brazed assembly, as well as to an assembly thus manufactured. The aluminum alloy is of the Aluminum Association 3xxx-type. Herein the term sheet material includes tube material, plate material and header material.
A principle use of brazing sheet containing such alloy is in heat exchangers, such as radiators, condensers and oil coolers. These heat exchangers are exposed to a severe external corrosive attack by e.g. deicing road salt. For that reason a good corrosion resistance is an essential property. Long-life alloys are considered herein as those which in the Sea Water Acidified Accelerated Test (SWAAT) without perforations according to ASTM G-85 exceed 10-12 days (see K. Scholin et al., VTMS 1993, SAE P-263). A further important property of the brazing sheet is the strength after brazing, hereafter referred to as the post-brazed strength.
WO 94/22633 describes such an alloy, having the composition, in weight %:
balance aluminum and impurities.
This alloy is used as core material with brazing clad layers containing Si. The high Cu content is to improve post-brazed strength. Ti is preferably not deliberately added, though is typically present from source material. Preferably Zr is not deliberately added. Cr and/or V are said not to improve post-brazed corrosion resistance, but contribute to post-brazed strength and sag resistance. The brazing sheet of WO 94/22633 has a reported post-brazed yield strength in the range of 54-85 MPa.
EP-A-0718072 discloses a brazing sheet having a core sheet made of an aluminum alloy core material and on at least one side thereof a brazing layer of an aluminum alloy containing silicon as main alloying element, wherein the aluminum alloy of the core sheet has the composition (in weight %):
balance aluminum and unavoidable impurities, and with the proviso that (Cu+Mg) greater than 0.7.
The disclosed core alloy has a Si-level of more than 0.15%, and most preferably of more than 0.40%, in order to achieve the desired strength levels while maintaining a good corrosion resistance.
EP-A-0537764 discloses a method of producing aluminum alloy heat-exchanger in which a brazed assembly after brazing is cooled and then reheated for 10 minutes to 30 hours at a temperature in the range of 400 to 500xc2x0 C. This additional heat treatment after brazing is in order to deposit elements (e.g. Si, Mg and Mn) which are brought into solid solution during the brazing cycle, and is said to improve the thermal conductivity of the material and thereby improving the thermal efficiency of the heat-exchanger obtained by about 3%. The core alloy used comprises not more than 0.5% of Cu and further comprises Si as an alloying element in the range of 0.05 to 1.0%.
U.S. Pat, No. 4,214,925 discloses a method for fabricating a brazed aluminum fin heat exchanger, in which the fins have a composition comprising 0.15 to 0.40 weight % of Cu, and is preferably of the heat-treatable AA6951 alloy, and in which the core sheet material of the brazing sheet is of the conventional AA3003 alloy. The cooling rate after solution heat-treatment for 30 minutes to 4 hours at 500 to 570xc2x0 C., is in the range of 2.8 to 50xc2x0 C./min, preferably 2.8 to 20xc2x0 C./min, and more preferably about 10xc2x0 C./min.
The later published international patent application no. PCT/EP97/06070 mentions a non-heat treatable aluminum alloy as core alloy in brazing sheet, i.e. it does not require post-brazing ageing treatment. Said aluminum core alloy, consisting of, in weight %:
balance aluminum and unavoidable impurities, and wherein 0.20xe2x89xa6(Cr+Zr)xe2x89xa60.4.
An object of the invention is to provide an aluminum alloy for use in a brazed assembly, in particular as core alloy in brazing sheet or as fin stock material, providing improved strength properties in combination with good corrosion resistance.
According to the invention, there is provided an aluminum alloy in the form of a sheet, plate or extrusion, having a composition in the range (in weight %):
and said aluminum alloy is provided in an aged condition.
In accordance with the invention it has surprisingly been found that the aluminum alloy appears to be age hardenable in the post-brazed condition, both by means of natural ageing and by artificial ageing. This ageing effect after brazing was yet undiscovered and is untypical for standard AA3xxx type alloys. It gives the possibility of a significant increase of the obtainable post-brazed yield strength in a range of 5 to 35 MPa over the post-brazed yield strength reported in the prior art, while the good corrosion resistance remains unchanged after the ageing treatment.
According to the invention the aluminum alloy is capable of providing a 0.2% yield strength of at least 75 MPa after brazing and ageing, and has a corrosion resistance of 13 days or more in SWAAT without perforations in accordance with ASTM G-85.
In a more preferred embodiment the aluminum alloy is capable of providing a 0.2% yield strength of at least 80 MPa after brazing and ageing, and more preferably of at least 85 MPa after brazing and ageing.
In the best examples, this corrosion resistance is more then 20 days. This level of corrosion resistance qualifies the alloy as a long-life product. Further, in the best examples, the provided 0.2% yield strength after brazing and the ageing is at least 95 MPa. Typically, but not by means of limitation, brazing is performed at about 590 to 600xc2x0 C. for 3 to 5 min.
The aluminum alloy is of the AA3xxx type, Mn being the main alloying element in order to obtain the desired strength level. At least 0.7% is required for obtaining the desired strength, while a Mn content of over 1.5% does not produce any significant improvements in respect strength because coarse Alxe2x80x94Mn-containing particles are formed. A further disadvantage of coarse Alxe2x80x94Mn-containing particles is that they reduce the rollability of the aluminum alloy. More preferably the Mn content is in a range of 0.8 to 1.2%.
Magnesium is used in core alloys for brazing sheet to improve strength in vacuum brazed products. If a flux brazing process is applied, the Mg content is preferably kept at a low level, and preferably lower than 0.4%. In a further embodiment a Mg content of zero is preferred in flux brazing processes in which the brazability is improved. The Mg content is specified as up to 0.8% maximum and preferably 0.5% maximum.
The Si content in the aluminum alloy of this invention should be less than 0.15% in order to obtain long-life corrosion performance, and is preferably less than 0.10%. In an even more preferred range the Si is present at impurity level. Despite the low Si content a significant ageing effect is observed.
The Cu content in the aluminum alloy increases the strength of the alloy and should be in the range of 0.5 to 1.5%, and is preferably larger than 0.7%. In particular in this range in combination with a low Si content and in combination with Mg, the unexpected ageing effect has been observed, while the desired long-life corrosion resistance does not decrease significantly. With a Cu content of over 1.5% undesired coarse Cu-containing particles can be formed, as well as low melting phases. Preferably the Cu content is not more than 1.2%. The appearance of the strong ageing effect at the relative dilute levels of Cu and MNg is regarded as unexpected.
Fe is present in all known commercial aluminum alloys but in the aluminum alloys in accordance with this invention it is not a required alloying element and is not deliberately added. With a high Fe content among other things the corrosion resistance decreases. The admissible Fe content is 0.4% maximum and preferably 0.25% maximum.
Zinc may be included, preferably in a range of 0.0 to 2.0%, so that it remains in solid solution and helps to lower the corrosion rate.
In an embodiment the aluminum alloy in accordance with the invention contains at least one element selected from the group consisting of from 0.05 to 0.30% of Cr, from 0.05 to 0.30% Ti, from 0.05 to 0.30% of Zr, and from 0.05 to 0.30% of V. The addition of at least one of the above mentioned elements results in at least a further improvement of the post-braze strength level after the ageing treatment. At contents above 0.25% of the individual elements undesired coarse particles can be formed.
The total amount of the optional additions of Cr, Ti, Zr, and V is chosen such that 0.05 less than (Cr+Ti+Zr+V) less than 0.4.
In another embodiment of the invention at least Zr is present in a range of 0.05 less than Zr less than 0.25%, and more preferably in a range of 0.05 less than Zr less than 0.15%. It has been found that Zr in particular improves the ageing response of the aluminum alloy and results in significant increases of the post-brazed and aged strength levels. In the best examples the yield strength after brazing and ageing is at least 95 MPa, which is an achievement over the post-brazed yield strength reported in the prior art.
In another preferred embodiment of the invention the aluminum alloy has a composition as mentioned in the international patent application no. PCT/EP97/06070, which is included here by reference. The composition of this aluminum alloy is (in weight %):
balance aluminum and unavoidable impurities, and wherein 0.20 less than (Cr+Zr) less than 0.4.
The invention also consists in brazing sheet comprising, as core material (i.e. strength providing material), the alloy of the invention described above. A clad or coating layer acting as a sacrificial anode in contact with water is not required, such a layer may be provided on one or both sides of the core alloy. On one side, in contact with the core alloy, there will normally be a clad layer in the form of a conventional low melting alloy filler layer.
The invention further consists in use of the aluminum alloy of the invention described above as core material of a brazing sheet in a brazed assembly. In such an assembly, the aluminum alloy core material may be directly in contact with the brazing alloy which is melted at the brazing temperature.
The invention further consists in use of the aluminum alloy of the invention described above as fin stock material in a brazed assembly.
Although they are particularly suitable for brazing purposes, the alloys of this invention are also capable of being extruded to yield corrosion resistant extruded sections.
The invention further consists in the use of an aluminum alloy having a composition (in weight %):
for subjecting to an ageing treatment after cooling from brazing where the cooling rate is at least in the range of typical brazing furnace cooling rates. Typical ageing treatments are natural ageing and artificial ageing. More preferred ranges for the alloying elements are set out above.
The invention also provides a method for manufacturing a brazed assembly using brazing sheet or fin stock material, comprising the steps of:
(i) forming parts of which at least one is made from the brazing sheet;
(ii) assembling the parts into the assembly;
(iii) brazing the assembly;
(iv) cooling the brazed assembly to below 100xc2x0 C. with a cooling rate of at least 20xc2x0 C./min;
(v) ageing the brazed and cooled assembly,
and wherein the brazing sheet has a core made of an aluminum alloy having the composition (in weight %):
In accordance with this invention it has been found that the cooling rate after the brazing cycles plays an important role in obtaining the yet undiscovered ageing effect after brazing. More preferably the cooling rate after brazing is at least 40xc2x0 C./min, and more preferably at least 60xc2x0 C./min. Increasing the cooling rate after the brazing cycles can give rise to a further increase in the strength levels which can be obtained. The appearance of the strong ageing effect after brazing at the relative dilute levels of Cu and Mg is regarded as unexpected, in particular since the brazing cycle is relatively short and no water quench is applied.
Typically ageing processes for obtaining the desired level of yield strength are (i) natural ageing, and (ii) artificial ageing at a temperature in the range of 100 to 250xc2x0 C. for a soaking time in a range of 5 to 1000 hours. The ageing treatment is discussed in more detail further below.
The invention also provides a brazed assembly comprising at least two members bonded together by means of a brazing alloy, at least one of the members being sheet material comprising the aluminum alloy of the invention described above as its core.
It should be mentioned here that in European patent application no. EP-A-0718072 a comparative example C7 is described containing in weight %: 1.1% Mn, 0.75% Cu, 0.5% Mg, 0.1% Si, balance essentially aluminum and impurities. In FIG. 1 of this publication it is shown that the alloy has an increase in 0.2% yield strength due to natural ageing after a simulated brazing cycle. However, in the description nothing is mentioned about the cooling rate after the simulated brazing cycle.